# How to measure 20AMP?

It varies with the battery chemistry. Alkalines for example have a steadily declining voltage as remaining capacity goes down, so you can get a fair indication of capacity from the voltage under load. Unfortunately most common rechargeable (NiMH, NiCad, Li-Ion) don't have the same convenient property. A NiMH for example will stay close to 1.2 volts till around 90% of its capacity is gone, then you get a pretty rapid decrease in voltage.
You can tell when a battery is almost empty by measuring the voltage and indeed you should to avoid over-discharging batteries, which can shorten their lives or even cause a fire in extreme circumstances (the big, unprotected, high-discharge Lithium battery packs used in RC flying can go up quite spectacularly).
You can also tell when a battery is fully charged /while it's being charged/ by measuring the voltage, peak-detect charging is one example - used for NiCad and NiMH cells (but not Lithium!).
But back to your question, 'smart' battery chargers measure the current going in and out. If you start with a fully charged 700mAh cell and you've been drawing 300mA for an hour you have 400mAh left. On batteries for laptops, phones and camcorders the circuitry to do this is often in the battery itself. In other words, they don't actually measure the remaining capacity, they estimate it based on known parameters and the current going in and out. It's not easy to get it exactly right (I've missed out a lot of the complexities) and you need a periodic full charge/discharge cycle to keep the electronics in synch with the real battery.
I get by with a low battery alarm based on the voltage and know roughly how long my 'bot will go without a recharge from experience.
Tim
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Clamp-around DC ammeters are now available at reasonable prices, under \$150. These are a big win when working with medium and high current DC circuits. They're useful down to about 1A. Note that most clamp-around meters are AC only, but DC ones are available. You usually get all the usual DVM functions built in, too.
If you want to build one yourself, look here:
http://www.siliconchip.com.au/cms/A_30669/article.html
John Nagle                 Team Overbot
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What`s wrong with shunting-off a small bit of the current? ----------------------------------------------------------------------- Ashley Clarke -------------------------------------------------------